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Tropical ocean sucks up vast amounts of ozone

An important mechanism for sucking ozone and methane out of the atmosphere has been discovered over the tropical Atlantic. The finding reveals how the two greenhouse gasses are kept in check by natural chemical reactions.

Researchers warn, however, that there is a risk the process could be overpowered by rising industrial pollution.

The data collected in Cape Verde, off the western coast of Africa, suggests that 50% more ozone is being destroyed above the tropical Atlantic Ocean than previously thought, because of halogens released by the seawater.

The process also increases the amount of methane pumped out of the lower atmosphere by up to 12% each day. It could occur in tropical oceans worldwide and could considerably improve the forecasts generated by the models used to study climate change.

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“At the moment this is a good news story&colon; more ozone and methane being destroyed than we previously thought,” says Alastair Lewis of the National Centre for Atmospheric Science in Leeds, UK.

“But the tropical Atlantic cannot be taken for granted as a permanent sink for ozone. The composition of the atmosphere is in fine balance here,” he adds.

Surprise result

Ozone in the troposphere, up to 10 kilometres above the Earth’s surface, is the third most important anthropogenic greenhouse gas after carbon dioxide and methane.

Yet climate models are not very precise when it comes to predicting where and in what quantities the gas is produced and removed. Part of the problem is that the molecule is largely recycled above the tropical oceans, where data is sparse.

Using the recently set up Cape Verde Atmospheric Observatory, Lewis and an international team have for the first time been able to study these processes in detail.

When they measured how the concentrations of ozone in the troposphere varied over time, they discovered that 13% of ozone is lost each day in this area, not 8% as predicted by models.

Sunny sink

Ozone is known to be largely broken down by sunlight and water vapour. This produces hydroxyl radicals, which in turn remove methane from the atmosphere.

Halogens like iodine and bromine can also help break down ozone. When the researchers plugged the bromine and iodine values measured at the observatory into their model they were better able to predict the decay of ozone. The results suggested that the halogens help create an ozone “sink”, which sucks the greenhouse gas out of the lower atmosphere.

“It has come as a surprise to find these chemicals, not only in coastal regions with lots of iodine rich seaweed, but also in the middle of the Atlantic ocean,” says Lewis.

He says bromine is released into the atmosphere by sea spray, while the main source of iodine above the open ocean is probably phytoplankton. These produce compounds containing iodine that are then broken down by sunlight.

“We have no reason to think that our study area is different from other tropical ocean regions, so similar ozone destruction could be happening on a huge global scale,” he says.

Fine balance

Neil Harris of the European Ozone Research Coordinating Unit at the University of Cambridge, UK, says it is important to include the role of halogens in climate models to understand their potential interaction with climate change.

“There is huge potential for these processes to be affected as global warming changes winds, water temperatures and ocean productivity,” he says. Changing winds would affect how much bromine is released in sea spray.

The researchers also warn that the precious greenhouse gas sink could be threatened by a class of chemicals that are coughed up by cars and factories. Nitric oxides boost the production of ozone, an effect which is currently overpowered by the halogen and sunlight-driven tropical sink.

“It will only take a small increase in nitrogen oxides from fossil fuel combustion, carried here from Europe, West Africa or North America on the trade winds, to tip the balance from a sink to a source of ozone,” explains Lewis. The Asian economies, especially China’s, are also a growing concern.